This invention relates to apparatus and methods for constructing and treating subterranean formations.
Typically, after a well for the production of oil or gas has been drilled, casing is lowered and cemented into the well bore. Normal primary cementing of the casing string in the well bore includes lowering the casing to a desired depth and displacing a desired volume of cement down the inner diameter of the casing. Cement is displaced downward into the casing until it exits the bottom of the casing into the annular space between the outer diameter of the casing and the well bore apparatus.
The casing may also be cemented into a well bore by utilizing what is known as a reverse-cementing method. The reverse-cementing method comprises displacing conventionally mixed cement into the annulus between the casing string and the annulus between an existing string, or an open hole section of the well bore. As the cement is pumped down the annular space, drilling fluids ahead of the cement are displaced around the lower ends of the casing string and up the inner diameter of the casing string and out at the surface. The fluids ahead of the cement may also be displaced upwardly through a work string that has been run into the inner diameter of the casing string and sealed off at its lower end. Because the work string has a smaller inner diameter, fluid velocities in the work string will be higher and will more efficiently transfer the cuttings washed out of the annulus during cementing operations. To ensure that a good quality cement job has been performed, a small amount of cement will be pumped into the casing and the work string. As soon as a desired amount of cement has been pumped into the annulus, the work string may be pulled out of its seal receptacle and excess cement that has entered the work string can be reverse-circulated out the lower end of the work string to the surface.
Reverse cementing, as opposed to the conventional method, provides a number of advantages. For example, cement may be pumped until a desired quality of cement is obtained at the casing shoe. Furthermore, cementing pressures are much lower than those experienced with conventional methods and cement introduced in the annulus free-falls down the annulus, producing little or no pressure on the formation. Oil or gas in the well bore ahead of the cement may be bled off through the casing at the surface. Finally, when the reverse-cementing method is used, less fluid is required to be handled at the surface and cement retarders may be utilized more efficiently.
The equipment required for reverse-cementing operations, like the equipment for the conventional method, is typically transported to the worksite via a number of tractor-trailers. Since the operation of tractor-trailers is highly regulated, the cementing operations are also controlled by Department of Transportation (“D.O.T.”) regulations. These regulations cover a number of variables, including the number of hours a driver may drive. This can lead to delay in operation, and may increase costs. For example, a driver may use up all his regulated working hours to get to the worksite and set up. As a result, he cannot do any more work that day. Since time is often critical in these operations, another worker must be present to do work that the driver could otherwise do. For example, a cementer may have the ability to drive the tractor-trailer. However, rather than drive a tractor-trailer to the worksite, set up, and cement, the cementer may be required to drive a personal car to the worksite, set up, and cement. In this scenario, a separate driver drives the tractor-trailer to the worksite. Since the driver's work includes driving, he may not even be able to drive to a hotel to sleep. Instead, he often must stay at the worksite (and on the clock) without working until enough time has passed and D.O.T. regulations permit him to work again. These regulations also control the skill level of the drivers. Only drivers having a special license may operate tractor-trailers. Since obtaining this type of license requires extensive training, drivers with specialized licenses are generally more expensive than drivers without such a license. Tractor-trailers are also limited by terrain, and may not be able to get to or enter certain worksites without suitable roads first being built, which may be a costly endeavor.
While the use of tractor-trailers keeps the cost of reverse-cementing operations high, this problem is not limited to reverse-cementing operations. The costs associated with the use of tractor-trailers extend to fracturing, or acid treatments, along with a number of other production enhancement operations.